The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for completing subterranean wells, in particular, fluid compositions and methods for completion operations during which the fluid compositions are pumped into a wellbore and make contact with subterranean rock formations.
In the course of completing oil and gas wells and the like, various types of fluids are circulated in the wellbore. These fluids include, but are not limited to, drilling fluids, spacer fluids, cement slurries and gravel-packing fluids. In addition, these fluids typically contain solid particles.
Cement slurries are frequently incompatible with most drilling fluids, particularly oil-based (OBM) and synthetic (SBM) fluids. If the cement slurry and drilling fluid commingle, a highly viscous mass may form that can cause several problems. In this application, commingling is defined as any direct contact between a drilling fluid and a cement slurry. Cement slurry can channel through the viscous mass, preventing bonding of the cement slurry to casing and formation surfaces. Unacceptably high friction pressures can develop during the cement job that may result in fracturing the formation or the development of lost circulation problems. If the viscous mass is sufficiently severe, the annulus may become plugged, terminating cement displacement. Any of these consequences would lead to job failure, compromising zonal isolation and possibly forcing operators to perform costly remedial operations. Plugging of the annulus can result in job failure. In each of these situations, zonal isolation may be compromised, and expensive remedial cementing may be required.
Consequently, intermediate fluids called preflushes are often pumped as buffers to prevent contact between cement slurries and drilling fluids. Preflushes can be chemical washes that contain no solids or spacer fluids that contain solids and can be mixed at various densities.
Spacers are preflushes with carefully designed densities and rheological properties. Spacers are more complicated chemically than washes. Viscosifiers may be employed in spacer fluids to suspend the solids and control the rheological properties. Examples of viscosifiers include water-soluble polymers or clays, or both. Other chemical components include dispersants, fluid-loss control agents, weighting agents, clays, antifoam agents, solvents and surfactants. A thorough discussion concerning the uses and compositions of preflushes may be found in the following publication. Daccord G, Guillot D and Nilsson F: “Mud Removal,” in Nelson E B and Guillot D (eds.): Well Cementing—2nd Edition, Houston: Schlumberger (2006) 183-187.
For optimal fluid displacement, the density and viscosity of a spacer fluid are carefully designed. Specifically, the density of a spacer fluid may be higher than that of the drilling fluid and lower than that of the cement slurry. Furthermore, the viscosity of the spacer fluid may be designed to be higher than the drilling fluid and lower than the cement slurry. The spacer fluid may remain stable throughout the circulation process (i.e., no free-fluid development and no sedimentation of solids). Such stability is particularly desired when the well is highly deviated or horizontal. In addition, the fluid-loss rate may be controlled.
Another function of preflushes is to leave the casing and formation surfaces water wet, thereby promoting optimal bonding with the cement. Achieving water-wet surfaces may be challenging, especially when the drilling fluid has been non-aqueous. Such non-aqueous fluids (NAF) may be oil-base muds or emulsion muds whose external phase is oil-base. For these circumstances, special dispersant, solvent and surfactant systems have been developed by the industry. Designing a dispersant/solvent/surfactant system for a particular well may be complicated because several parameters are considered, including the base oil of the NAF, the presence of emulsifiers, the fluid density, bottomhole temperature, presence of brine salts and the chemical nature of the cement system.